1. Field of the Invention
The present invention relates generally to transistor breakdown protection circuits and, more particularly, to a circuit for buffering an output voltage including a low-power integrated back buffer that limits the reverse-biasing of disabled transistors in the circuit.
2. Discussion of Related Art
A bipolar transistor, when correctly biased, may provide signal amplification between, for example, its base and its collector. One variable that measures such base-to-collector signal amplification is the current amplification factor (.beta.), i.e., the ratio of collector current to base current, of the transistor. In normal operation, a bipolar transistor typically will have its base-to-emitter junction forward-biased by at least (approximately) 0.7 volts and will have its base-to-collector junction reverse-biased such that, when a signal is placed at the base node, an amplified signal will be provided at one of its current nodes, i.e., its collector or emitter. If, however, a p-n junction of a transistor is reverse-biased by a voltage above a particular threshold voltage, damage to the transistor may result. For example, if the emitter-base junction of a transistor is reverse-biased by too high a voltage: (1) a phenomena known as breakdown may occur at the junction when the reverse-bias voltage exceeds the emitter-base breakdown voltage (BVebo), or (2) the current amplification factor (.beta.) may be degraded (i.e., .beta. degradation), which typically occurs at voltages substantially less than BVebo.
Occasionally, bipolar transistors are used in circuits wherein the (amplifying) transistors are disabled temporarily, e.g., when a biasing signal that maintains a transistor in its linear region is removed, but wherein the transistor still receives a fluctuating signal at one of its nodes. For example, in a circuit including several buffers having their outputs connected to form a single output node (wherein only one of the buffer circuits is enabled at any given time), the output transistors in all but one of the buffer stages would be disabled, while these output transistors still would be influenced by the output signal of the selected stage. In such circuits, the potential for emitter-base breakdown (or .beta. degradation) of the output transistors in the unselected stages is a cause for concern.
U.S. Pat. No. 5,179,293 (hereinafter "the Hilton patent") owned by Analog Devices, Inc., discloses one circuit for limiting the reverse-biasing of the base-emitter junctions of transistors in an output stage when the output stage is disabled (but still is influenced by a signal at the output node of the output stage). The circuit disclosed in the Hilton Patent includes a unity-gain complementary emitter-follower (with a characteristically high input impedance) coupled to the output node of the circuit to receive whatever signal is present at the output node. When the output stage is enabled, this output node is driven from the emitters of pair of (large) complementary output-driver transistors. The output of the complementary emitter-follower is switchably connected (via a pair of switching-transistors) to the bases of the pair of output-driver transistors. The pair of switching-transistors is controlled so as to provide the output of the emitter-follower (in addition to a base-emitter voltage drop across each of the pair of switching-transistors) to the bases of the pair of output-driver transistors only when the output stage is disabled. In this manner, when the output stage is disabled, the emitter-base junction of each of the output-driver transistors has a reverse-bias voltage equal to one base-to-emitter voltage drop (Vbe) maintained thereacross.
The circuit in the Hilton patent also includes complex circuitry to minimize the output leakage current that emanates from the complementary emitter-follower when the output stage is disabled. This additional leakage compensation circuitry, as well as the biasing circuitry of the complementary emitter-follower, consume power even when the output stage is enabled and driving a load. Thus, while it is able to switch between enable and disable modes relatively quickly, the Hilton circuit is not extremely power efficient.
Further, because the output-driver transistors in the Hilton circuit are relatively large, the are plagued with substantial base-to-collector and base-to-emitter parasitic capacitances. To attain and maintain the desired reverse-bias voltages across the base-emitter junctions of these (large) drivers, then, a substantial transient charge is required to overcome this parasitic capacitive loading. Additionally, since in disable mode the (large) output devices in the Hilton circuit are not merely disabled, but rather, are reverse-biased by a voltage of one Vbe (i.e., approximately 0.7 volts), a transient charge large enough to cause this (larger than necessary) voltage change across the parasitic base-to-emitter capacitance is required. The Hilton circuit supplies the transient charge (required to charge the parasitic capacitances of its output devices) via its emitter-follower circuit (driven by the output). This transient charge supplying scheme requires active devices, which may consume a significant amount of power, to charge the (large) output devices. Also, an increase in the switching speed of the Hilton circuit necessarily would require an increase in the amount of power consumed by its active transient charge supplying devices.
It therefore is a general aim of the present invention to provide an output stage that protects its output devices (when disabled) from excessive reverse-biased voltages, but that is simpler than prior art circuits and does not consume a significant amount of power in its disabled state. It is an additional aim of the invention to provide an output stage that protects its output devices (when disabled) and is capable of switching at higher speeds (while consuming less power) than prior art circuits.